Measuring concentration is not an easy task in engineering and process practice. Most concentration (or analysis) sensors are designed specifically for certain type of species. Other general methods, such as chromatography and spectroscopy, require sampling and laboratory analysis hence are not suitable for on-line measurement of the concentration.
Some works that measure concentration using indirect measurement variables can be listed as follows:                Concentration distribution in a fluid by electric impedance tomography [Ijaz et. al., 2007]        Lactic acid concentration estimation in fermentation broth by conductivity measurement, [Payot et. al., 1997]        Method for determining component concentration in three-component mixture, [Sota et. al., 2002], U.S. Pat. No. 6,350,426.        Method and apparatus for analyzing chemical composition, [Markant et. al., 1970], U.S. Pat. No. 3,537,820        Arrangement for determining concentration of measurement liquid with neural network based on conductivity, [Schwulera and Riegel, 2000], German Patent Office, DE 198 35 137 A 1.        
Out of the previous methods, numbers 2 and 5 use conductivity, however in a non-dynamic way, and based on a direct data-driven inference, which makes them radically different from the approach taken in this invention.
When the task of measuring concentration is focused in a chemical reactor, it becomes even more challenging, since concentration exhibits a dynamic behavior that most sensors are not capable of taking into account.
So far, common production practices and technology call for off-line sample analysis from the product stream, and the reacting volume is monitored for operational variables such as temperature and pressure, but not for product quality purposes. Product stream analysis is performed by sampling every often and taking those samples to a laboratory.
Lately, some estimation methods have been developed to monitor a progressing reaction based on the use of in-situ viscometer [Elli, et. al., 2007] and fiber-optic near-infrared (NIR) spectroscopy [Knothe, 2000]. Later, the NIR method by Knothe was used to measure biodiesel content in a hydrocarbons mixture, useful in blending applications, as presented on U.S. Pat. No. 7,404,411. Using a viscometer requires adding a moving element into the reaction vessel. NIR spectroscopy is a very expensive technology, not always suitable for the plant floor, and it also requires a well trained operator. Also, the methods presented in the literature, do not explicitly take into account dynamic behavior.
An unfulfilled need addressed by the present invention is to provide an automated method and a non-moving apparatus to measure species concentration in the reaction vessel based on a variable measurement easily understood by the process operators and technicians.